Methods of Using Fluid Loss Additives Comprising Micro Gels

ABSTRACT

Methods and fluids are provided that include, but are not limited to, a drilling fluid comprising an aqueous base fluid and a fluid loss control additive that comprises at least one polymeric micro gel and a method comprising: providing an aqueous based treatment fluid comprising a fluid loss control additive that comprises at least one polymeric micro gel; placing the aqueous based treatment fluid in a subterranean formation via a well bore penetrating the subterranean formation; allowing the fluid loss control additive to become incorporated into a filter cake located on a surface within the subterranean formation; allowing the filter cake to be degraded; and producing hydrocarbons from the formation. Additional methods are also provided.

BACKGROUND

The present invention relates to methods of using fluid loss controladditives. More specifically, at least in some embodiments, the presentinvention relates to the use of fluid loss control additives in drillingand servicing fluids that comprise polymeric micro gels in subterraneanoperations.

When well bores are drilled into producing formations, drilling fluidsare utilized which will minimize damage to the permeability of theformations and their ability to produce hydrocarbons. Servicing fluidsare utilized when completion operations are conducted in producingformations and when conducting work-over operations in the formations.The drilling and servicing fluids deposit a layer of particles known as“filter cake” on the walls of the well bores within the producingformations. The filter cake is believed to help prevent the drilling andservicing fluids from being lost into the formation and prevents solidsfrom entering the porosities of the rock. Following completion and priorto initiating production, the filter cake is usually degraded or allowedto degrade to allow product to flow into the well bore for production.Degrading the filter cake is important to retain well bore connectivityand the natural permeability of the reservoir rock. If not degraded orallowed to degrade, the filter cake could present an impediment toproduction, inter alia, by altering the permeability of the reservoir.Once the permeability of the reservoir has been diminished, it is seldomable to restore it to its original condition. These should bedistinguished from the function of additives (sometimes termed in theart “relative permeability modifiers” or “RPMs”) that are often used inconformance or fracturing fluids to permanently seal water influxes tohydrocarbon reservoir areas surrounding a well bore.

Drilling and servicing fluids (such as drill-in fluids) may comprisefluid loss control additives to further assist in preventing thedrilling and servicing fluids from being lost into the formations.Drilling fluids are any of a number of fluids and mixtures of fluids andsolids (as solid suspensions, mixtures and emulsions of liquids, gasesand solids) used in operations to drill boreholes into the earth.Classifications of drilling fluids has been attempted in many ways,often producing more confusion than insight. One classification scheme,given here, is based only on the mud composition by singling out thecomponent that clearly defines the function and performance of thefluid: (1) aqueous-based, (2) oil-based and (3) gaseous (pneumatic).Each category has a variety of subcategories that overlap each otherconsiderably. Ideally, a drilling fluid is non-damaging to theformation, meaning that the fluid does not leave behind particulates,fines, etc. that negatively impact the permeability of the formation.

A drill-in fluid is a fluid designed for drilling through the reservoirsection of a well bore in a subterranean formation. The reasons forusing a specially designed fluid include, but are not necessarilylimited to: (1) to drill the reservoir zone successfully, often a long,horizontal drain hole; (2) to minimize damage to and maximize productionof exposed zones; and (3) to facilitate the well completion needed,which may include complicated procedures. A drill-in fluid oftenresembles a completion fluid in that it may comprise a brine, possiblybridging agents, and/or polymers.

The term “drilling fluid” as used herein refers generically to bothdrilling fluids and drill-in fluids unless otherwise specified.

Other types of treatment fluids that can utilize fluid loss controlmaterials include, but are not limited to, pills (such as inside screenpills), which are fluids with a relatively small quantity (e.g., lessthan 200 bbl) of a special blend of drilling fluid to accomplish aspecific task that the regular drilling fluid cannot perform. Examplesinclude high-viscosity pills to help lift cuttings out of a verticalwell bore, freshwater pills to dissolve encroaching salt formations,pipe-freeing pills to destroy filter cake and relieve differentialsticking forces, and lost circulation material pills to plug a thiefzone. Another example is a screen pill that may be useful in conjunctionwith a gravel pack operation.

Examples of conventional fluid loss control additives for water-basedtreatment fluids include nonionic water soluble polymers, such asstarches, derivatized starches, gums, derivatized gums, and cellulosics.Fluid loss additives that include starches often vary in the ratio ofamylose to amylopectin content, and may or may not be modified with acrosslinking agent such as epichlorohydrin. Also, natural starches maynot be uniform in terms of quality and effectiveness. These cross-linkedstarches often do not have thermal stability at temperatures up to about250° F., and at temperatures above 250° F., they can only effectively beused by increased loading the treatment fluid with the cross-linkedstarch, constantly replenishing the treatment fluid with thecross-linked starch, or using an oxygen scavenger in conjunction withthe cross-linked starch. At temperatures above 300° F., even by the useof the above mentioned measures, cross-linked starches may not beeffective fluid loss control additives.

Conventional linear synthetic polymers are also utilized, butoftentimes, they require another additive, such as a clay, to be able toeffectively function as fluid loss control additives. However, the useof clay can be problematic in drill-in fluids, as removing the clay fromthe subterranean formation can be difficult because it infiltrates intopores of the subterranean formation. Furthermore, the addition of clayto a treatment fluid dramatically increases the viscosity of the fluid,which can cause drilling to be completed at a reduced rate. Anotherissue is that some synthetic polymers cannot be used successfully inconjunction with some brines, for instance, divalent brines. This brineincompatibility is thought to prevent the synthetic polymer fromassociating with the clay to forming bridging colloids, which aredesirable because they provide a degree of fluid loss, e.g., in adrill-in fluid.

SUMMARY

The present invention relates to methods of using fluid loss controladditives. More specifically, at least in some embodiments, the presentinvention relates to the use of fluid loss control additives in drillingand servicing fluids that comprise polymeric micro gels in subterraneanoperations.

In one embodiment, the present invention provides a drilling fluidcomprising an aqueous base fluid and a fluid loss control additive thatcomprises at least one polymeric micro gel.

In another embodiment, the present invention provides a methodcomprising: providing an aqueous based treatment fluid comprising afluid loss control additive that comprises at least one polymeric microgel; placing the aqueous based treatment fluid in a subterraneanformation via a well bore penetrating the subterranean formation;allowing the fluid loss control additive to become incorporated into afilter cake located on a surface within the subterranean formation;allowing the filter cake to be degraded; and producing hydrocarbons fromthe formation.

In another embodiment, the present invention provides a methodcomprising: providing a drilling fluid comprising an aqueous base fluidand a fluid loss control additive that comprises at least one polymericmicro gel; circulating the drilling fluid in a subterranean formation;allowing the fluid loss control additive to become incorporated into afilter cake located on a surface within the subterranean formation;allowing the filter cake to be degraded; and producing hydrocarbons fromthe formation.

In another embodiment, the present invention provides a methodcomprising: providing an inside screen pill comprising an aqueous basefluid and a fluid loss control additive that comprises at least onepolymeric micro gel, and placing the inside screen pill in asubterranean formation so that the inside screen pill is located betweena gravel pack screen and an area of the subterranean formation.

The features and advantages of the present invention will be readilyapparent to those skilled in the art. While numerous changes may be madeby those skilled in the art, such changes are within the spirit of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention, and should not be used to limit or define theinvention.

FIG. 1 is a photograph of a transparent solution of a micro geldispersion after dissolving in water.

FIG. 2 is a micrograph of the micro gel dispersion of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to methods of using fluid loss controladditives. More specifically, at least in some embodiments, the presentinvention relates to the use of fluid loss control additives in drillingand servicing fluids that comprise polymeric micro gels in subterraneanoperations.

There may be several potential advantages to the methods andcompositions of the present invention, only some of which are alluded toherein. The present application provides drilling fluids, preferably foruse as a drilling, drill-in, completion fluid or pills, which quicklyforms a thin, degradable filter cake. Again, for the sake of clarity,the term “drilling fluid” as used herein refers generically to bothdrilling fluids and drill-in fluids unless otherwise specified. Thedrilling fluids of the present invention are less invasive to theformation yet provide lubricity, effective fluid loss control, and goodfilter cake sealing and plastering characteristics. The resulting filtercake can be easily lifted off and readily removed or dissolved to helpensure low skin and residual damage. Moreover, the fluids are stable forprolonged periods of time at high temperatures. In particular, thedrilling fluids of the present invention are believed to have enhancedperformance over conventional fluids that comprise starch derivatives,for instance, because they can be used at effective static bottom holetemperatures up to at least about 400° F. or more while maintainingeffective fluid loss control, thus minimizing formation damage. In doingso, a vast range of applications is open for technological advancementand new product revenue. It is believed that in equal concentrations,the drilling fluids of the present invention that comprise fluid lossadditives comprising polymeric micro gels described herein give anincreased performance (e.g., an order of magnitude increase) in theamount of fluid loss over starch-based fluid loss materials.

Another potential advantage may be that the drilling fluids of thepresent invention may be used without the addition of clay, which allowsfor easier clean up, minimal formation damage, faster drilling times,and the avoidance of problems associated with the increased viscosity offluids when clay is added.

In some embodiments, the present invention provides an aqueous-baseddrilling fluid that comprises a fluid loss control additive of thepresent invention that comprises at least one polymeric micro gel. Inother embodiments, the present invention provides drilling fluids thatcomprise an aqueous base fluid (e.g., a brine or fresh water), and oneor more fluid loss control additives that comprise at least onepolymeric micro gel. In practical usage, with respect to building afilter cake that will easily degrade, it may be beneficial to includebridging agents in the drilling fluid that will become incorporated intothe filter cake. Without the bridging agents, the polymeric micro gelsmay form an effective filter cake, but that filter cake may be moredifficult to remove. Other components of the drilling fluid may compriseweighting agents, clays, polymers, as well as other additives common toaqueous-based drilling fluids including, but not necessarily limited to,lubricants, corrosion inhibitors, other inhibitors, and oxygenscavengers.

In some embodiments, the fluid loss control additive may be included inthe drilling fluid in an amount of about 5% or below by weight of activecomponent of the polymeric micro gel.

The base fluid of the drilling fluids of the present invention may beany suitable aqueous-based fluid including, but not necessarily limitedto, brines (i.e., water comprising a salt). In some instances, brineshaving a density of at least about 9 lb/gal, referred to as “highdensity brines,” may be suitable. The brines may contain substantiallyany suitable salts, including, but not necessarily limited to, saltsbased on metals, such as calcium, magnesium, sodium, potassium, cesium,zinc, aluminum, and lithium. Salts of calcium and zinc are preferred.The salts may contain substantially any anions, with preferred anionsbeing less expensive anions including, but not necessarily limited tochlorides, bromides, formates, acetates, and nitrates. Generally, thebase fluid is present in a drilling fluid of the present invention in anamount in the range of from about 35% to about 97% by weight thereof,more preferably from about 70% to about 95% or more.

Preferably, when used, the bridging agents are either self-degrading ordegradable in a suitable clean-up solution (e.g., a mutual solvent,water, an acid solution, etc.). Examples of bridging agents suitable foruse in the methods of the current invention include, but are notnecessarily limited to, magnesium citrate, calcium citrate, calciumsuccinate, calcium maleate, calcium tartrate, magnesium tartrate,bismuth citrate, calcium carbonate, sodium chloride and other salts, andthe hydrates thereof. Examples of degradable bridging agents mayinclude, but are not necessarily limited to, bridging agents comprisingdegradable materials such as degradable polymers. A polymer isconsidered to be “degradable” herein if the degradation is due to, interalia, chemical and/or radical process such as hydrolysis, oxidation,enzymatic degradation, or UV radiation. Suitable examples of degradablepolymers that may be used in accordance with the present inventioninclude, but are not necessarily limited to, those described in thepublication of Advances in Polymer Science, Vol. 157 entitled“Degradable Aliphatic Polyesters” edited by A. C. Albertsson, thedisclosure of which is hereby incorporated by reference. Specificexamples of suitable polymers include, but are not necessarily limitedto, polysaccharides such as dextrans or celluloses; chitins; chitosans;proteins; orthoesters; aliphatic polyesters; poly(lactides);poly(glycolides); poly(ε-caprolactones); poly(hydroxybutyrates);poly(anhydrides); aliphatic polycarbonates; poly(orthoesters);poly(amino acids); poly(ethylene oxides); and polyphosphazenes.Combinations and derivatives of these are suitable as well.

When choosing a particular bridging agent to use, one should be aware ofthe performance of that bridging agent at the temperature range of theapplication. The bridging agents utilized may be generally present inthe drilling fluid compositions in an amount in the range of from about1% to about 40% by weight thereof, more preferably from about 5% toabout 25%. Generally, the bridging agents may have a particle size inthe range of from about 1 micron to about 600 microns. Preferably, thebridging particle size is in the range of from about 1 to about 200microns but may vary from formation to formation. The particle size usedis determined by the pore throat size of the formation.

The term “polymeric micro gel” as used herein refers to a gelledparticle comprising a cross-linked polymer (e.g., water-soluble andwater-swellable) network. In some embodiments, the fluid loss controladditive may be delivered in a continuous medium, e.g., in a dispersionform. In other embodiments, the polymeric micro gels may be used withoutany continuous medium, e.g., in dry form. One of the many usefulfeatures of certain embodiments of the polymeric micro gels of thepresent invention is that they are believed to swell or shrink inresponse to an external stimuli, e.g., pH and temperature. In someembodiments, some of the polymeric micro gels may be considered to besuper absorbent in that they can contain over 99% water. This may beadvantageous in some applications wherein water absorbance is important.

When comprising a substantial portion of the fluid loss control additivein a drilling fluid of the present invention, the polymeric micro gelsoptionally may be dispersed in a continuous medium before addition tothe fluid. In some embodiments, the fluid loss control additive maycomprise a continuous medium. Examples of suitable continuous mediumsmay include, but are not necessarily limited to, aqueous-based fluids,alcohols, glycerin, glycols, polyglycol amines, polyols, and anyderivative thereof. Additionally, in some embodiments, the continuousmedium may comprise a fluid selected from the group consisting ofmethanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol,isobutanol, t-butanol, a mixture of methanol, ethanol, n-propanol,isopropanol, n-butanol, sec-butanol, isobutanol, or t-butanol and water,a mixture of ammonium sulfate, sodium sulfate, or potassium sulfate andwater, a mixture of sodium chloride, potassium chloride, or calciumchloride and water, and combinations thereof. Optionally, the continuousmedium comprises a fluid selected from the group consisting of ethanol,a mixture of t-butanol and water, and a mixture of ammonium sulfate andwater. Mixtures of these may be suitable as well. Examples of suitableaqueous-based fluids may include, but are not necessarily limited to,fresh water, sea water, salt water, and brines (e.g., saturated saltwaters). Examples of suitable brines may include, but are notnecessarily limited to, heavy brines, monovalent brines, divalentbrines, and trivalent brines that comprise a soluble salts like sodiumchloride, calcium chloride, calcium bromide, zinc bromide, potassiumcarbonate, sodium formate, potassium formate, cesium formate, sodiumacetate, potassium acetate, calcium acetate, ammonium acetate, ammoniumchloride, ammonium bromide, sodium nitrate, potassium nitrate, ammoniumnitrate, ammonium sulfate, calcium nitrate, sodium carbonate, potassiumcarbonate, any combination thereof, and any derivative thereof. Examplesof suitable alcohols may include, but are not necessarily limited to,methanol, ethanol, propanol, iso-propanol, butanol, tert-butanol, andthe like. Examples of suitable glycols may include, but are notnecessarily limited to, polyglycols, propylene glycol, ethylene glycol,and the like. In some embodiments, the continuous medium may be presentin any suitable range including ratios of continuous medium to polymericmicro gel of about 30:70, about 20:80, about 10:90, about 5:95, andabout 0:100 by weight.

In some embodiments, the polymeric micro gels may comprise a reactionproduct formed by a suitable polymerization reaction of a polymer (ormonomer) and a crosslinking agent. In one embodiment of a method offorming the micro gels of the present invention, the polymerization is adispersion polymerization in a continuous medium that is substantiallyinert toward chain transfer reactions. Crosslinked polymer micro gelparticles are formed, which are believed to be insoluble or at mostswellable in the continuous medium. While not wishing to be limited toany particular theory, it is believed that in some embodiments, thecrosslinking agent may act as an initiator forming chain branches of thepolymer or monomer, which in turn may react with one another to form apolymeric micro gel. Other suitable initiators may include those knownto those skilled in the art such as photoinitiators, thermal initiators,and combinations thereof.

In some embodiments, the polymer (or monomer) may comprise any suitablepolymer or monomer that can form a polymeric micro gel when crosslinked.Suitable polymers and monomers may include, but are not necessarilylimited to, those that comprise units based on acrylamides, vinylalcohols, vinylpyrrolidones, vinylpyridines, acrylates, polyacrylamides,polyvinyl alcohols, polyvinylpyrrolidones, polyvinylpyridines,polyacrylates, polybutylene succinate, polybutylenesuccinate-co-adipate, polyhydroxybutyrate-valerate,polyhydroxybutyrate-covalerate, polycaprolactones, polyester amides,polyethylene terephthalates, sulfonated polyethylene terephthalate,polyethylene oxides, polyethylenes, polypropylenes, aliphatic aromaticcopolyester, polyacrylic acids, polysaccharides (such as dextran orcellulose), chitins, chitosans, proteins, aliphatic polyesters,polylactic acids, poly(glycolides), poly(ε-caprolactones), poly(hydroxyester ethers), poly(hydroxybutyrates), poly(anhydrides), polycarbonates,poly(orthoesters), poly(amino acids), poly(ethylene oxides),poly(propylene oxides), poly(phosphazenes), polyester amides,polyamides, polystyrenes, any derivative thereof, any copolymer,homopolymer, or terpolymer, or any blend thereof. In some embodiments,the monomer may include an unsaturated group, such as a monomerincluding a vinyl group. Exemplary vinyl-containing monomers may bedescribed by the formula C(R₁)(R₂)═C(R₃)(R₄), wherein R₁, R₂, R₃ and R₄are segments rendering the solubility or swellability of this monomer inthe common solvent. Optionally, R₁, R₂, R₃ and R₄ can each beindependently selected from, but not limited to, hydrogen, methyl,ethyl, CONH₂, CONHCH₃, CON(CH₃)₂, CH₂SO₃H, CH₂SO₃Na and COONa. In afurther option, the monomer comprises a compound selected from the groupconsisting of hydroxyethyl acrylate, acrylamide and hydroxyethylmethacrylate. Examples of suitable copolymers that may be used withthese may include inorganic and organic polymers. Some of these polymersand monomers may be more suited for less high temperature applicationsthan others; however, crosslinking may increase their thermal stability.In some embodiments the polymer or monomer may comprise a water solublepolymer or monomer. In some embodiments the polymer or monomer maycomprise a block copolymer with a portion that is water soluble and aportion that is water insoluble.

In some embodiments, the crosslinking agent may comprise anycrosslinking agent that can react with a polymer or monomer to form apolymeric micro gel. In some embodiments, the crosslinking agent maycomprise a covalent crosslinking agent. Examples of suitablecrosslinking agents may include crosslinking agents that form a radical.Suitable crosslinking agents may include, but are not limited to,2,2′-azobis-(2-methylbutyronitrile), 2,2′-azobis(isobutyramidinehydrochloride),2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,1,1′-azobis(cyclohexanecarbonitrile),2,2′-azobis(2-methylpropionamidine) dihydrochloride,4,4′-azobis(4-cyanovaleric acid), ammonium persulfate,hydroxymethanesulfinic acid monosodium salt dihydrate, potassiumpersulfate, sodium persulfate, benzoyl peroxide,1,1-bis(tert-amylperoxy)cyclohexane,1,1-bis(tert-butylperoxy)cyclohexane, 2,2-bis(tert-butylperoxy)butane,2,4-pentanedione peroxide, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane,2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne; 2-butanone peroxide,cumene hydroperoxide, di-tert-amyl peroxide, dicumyl peroxide, lauroylperoxide, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butylperoxide, tert-butyl peroxybenzoate, tert-butylperoxy-2-ethylhexylcarbonate, diethylene glycol dimethacrylate, azobisisobutyronitrile, andcombinations thereof. A person of ordinary skill in the art would beable to select an appropriate crosslinking agent based upon the polymeror monomer to be crosslinked.

In some embodiments, the amount of crosslinking agent used inconjunction with the polymer or monomer may be any amount necessary tocreate the desired degree of crosslinking, and other characteristics. Insome embodiments, the ratio of crosslinking agent to monomer or polymerused to form the polymeric micro gel may be in the range of from about100:1 to about 5,000:1 by weight.

In one embodiment of a method of forming the micro gels of the presentinvention, the polymerization is a dispersion polymerization in acontinuous medium such as a continuous medium comprising t-butylalcohol/water and ammonium sulfate. Both the monomer (such as, but notlimited to, acrylamide) and the crosslinking agent (such as, but notlimited to, N,N′-ethylene-bis-acrylamide) are substantially soluble orat least swellable in the continuous medium. The resultant micro gelsare not soluble in the continuous medium. The continuous medium isbelieved to be relatively inert (or at least not so reactive as todisrupt micro gels from forming) toward chain transfer reactions bypropagating radicals. By removing the continuous medium using a suitabletechnique, the micro gels may be dried if desired.

Optionally, a colloidal stabilizer may also be included in thecontinuous medium during polymerization. One example of a colloidalstabilizer is amphiphilic, such as an amphiphilic colloidal stabilizercomprising a stabilizer selected from the group consisting of poly(vinylpyrrolidone) (PVP), polydiallyldimethylammonium chloride (poly-DADMAC),and combinations thereof.

In one embodiment of a method of forming the micro gels of the presentinvention, the polymerization is a dispersion polymerization ofwater-soluble or swellable vinyl monomer(s) (as represented byC(R1)(R2)=C(R3)(R4), where R1, R2, R3 and R4 are segments rendering thesolubility or swellability of this monomer to water and thepolymerization medium) in the presence of radically polymerizablecrosslinking agent (such as, but not limited to,C(R5)(R6)=C(R7)-R8-C(R9)=C(R10)(R11), where R5-R11 are groups renderingsolubility and swellability of this crosslinking agent to water and thepolymerization medium) in a medium (such as, but not limited to,t-butanol/water and ammonium sulfate solution in water), to which thechain transfer of propagating radical is avoided or suppressed. Bytaking advantage of the poor stability of the resultant dispersion athigh polymer content (e.g., 13 wt %), in the absence of any crosslinkingagent, the result may be one chunk of macro gel.

The size range for these polymeric micro gels may range from about 10 nmto about 1 mm in diameter. A suitable size range may be 10 microns toabout 500 microns. In a given composition, a range of disparate sizedpolymeric micro gels may be used. One of ordinary skill in the art wouldbe able to select an appropriate size of the polymeric micro gels basedupon the characteristics of the subterranean formation in which thepolymeric micro gels would be used, the temperature of the subterraneanformation, the degree of stability required of the polymeric micro gels,and the costs. Also, the shapes of the polymeric micro gels may vary,and may depend on several factors including, but not necessarily limitedto, temperature, the continuous medium used, and the degree ofcrosslinking. The polymeric micro gels can be any shape, for example, atleast partially spherical. Depending on the method in which thepolymeric micro gels are created, the shape of the polymeric micro gelscan vary. One of ordinary skill in the art would be able to select anappropriate shape of the polymeric micro gels based upon thecharacteristics of the subterranean formation in which the polymericmicro gels would be used, the temperature of the subterranean formation,the degree of stability required of the polymeric micro gels, and thecosts. No particular shape is critical for use in the present invention.

In some embodiments, the fluid loss control additives of the presentinvention may not be delivered with a continuous medium. These may bereferred to as the “dry” embodiments. For example, in some embodiments,after the polymeric micro gels are polymerized, the polymeric micro gelsmay be removed from the continuous medium and then dried using asuitable technique to form the powdered polymeric micro gels.

In some embodiments, the present invention comprises an aqueous baseddrill-in fluid comprising a fluid loss control additive of the presentinvention that comprises at least one polymeric micro gel. In someembodiments, a fluid loss control additive may be included in thedrill-in fluid in an amount of about 5% or below by weight of activecomponent of the polymeric micro gel. Other components of the drill-influids may comprise bridging agents and other commonly used additives.

In some embodiments, the present invention comprises an inside screenpill comprising a water-based fluid and a fluid loss control additivecomprising at least one polymeric micro gel. In some embodiments, fluidloss control additive may be included in the inside screen pill in anamount of about 1 to about 10% by weight of active component of thepolymeric micro gel.

In some embodiments, the present invention provides a drilling fluidcomprising an aqueous base fluid and a fluid loss control additive thatcomprises at least one polymeric micro gel.

In some embodiments, the present invention provides a method comprising:providing an aqueous based treatment fluid comprising a fluid losscontrol additive that comprises at least one polymeric micro gel; andplacing the aqueous based treatment fluid in a subterranean formationvia a well bore penetrating the subterranean formation.

In some embodiments, the present invention provides a method comprising:providing a drilling fluid comprising an aqueous base fluid and a fluidloss control additive that comprises at least one polymeric micro gel;and circulating the drilling fluid in a subterranean formation.

In some embodiments, the present invention provides a method comprising:providing an inside screen pill comprising an aqueous base fluid and afluid loss control additive that comprises at least one polymeric microgel, and placing the inside screen pill in a subterranean formation sothat the inside screen pill is located between a gravel pack screen andan area of the subterranean formation.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, theentire scope of the invention.

EXAMPLES Example 1

Polymeric micro gel particles were prepared in accordance with thefollowing procedure. A mixture of 95.0 g of ethanol, 20.0 g ofacrylamide, 0.50 g of poly(vinyl pyrrolidone), and 0.50 g of diethyleneglycol dimethacrylate was prepared in a 250 mL three-neck flask equippedwith a condenser and a mechanical stirrer. The mixture was purged withnitrogen gas for 30 minutes and then heated to 65° C. A 5.0 g solutionof azobisisobutyronitrile in ethanol (1% azobisisobutyronitrile byweight) was then injected into the mixture. The mixture was then kept at65° C. under nitrogen and stirred for 22 hours at 300 rpm. The mixturewas then allowed to cool to room temperature and a dispersion ofparticles with sizes from 10 μm to 100 μm (in water) was obtained withsolids content of approximately 17% by weight. FIG. 1 is a photograph ofthe transparent micro gel dispersion formed in this example. FIG. 2 is amicrograph of the micro gel dispersion formed in this example. Thedispersions were then centrifuged at 4800 rpm for 5 minutes. Thesupernatant was then decanted and sediments were collected. Thesediments were then derived under vacuum at approximately 50° C.overnight to generate loose, well dispersed dried powders.

Example 2

Polymeric micro gel particles were prepared in accordance with thefollowing procedure. A mixture of 95.0 g of a mixture of t-butanol andwater (90% t-butanol by weight), 15.0 g of acrylamide, 0.20 g ofN,N′-ethylene-bis-acrylamide, and 0.50 g of poly(vinyl pyrrolidone) wasprepared in a 250 mL three-neck flask equipped with a condenser and amechanical stirrer. The mixture was purged with nitrogen gas for 30minutes and then heated to 65° C. A 5.0 g solution ofazobisisobutyronitrile in ethanol (1% azobisisobutyronitrile by weight)was then injected into the mixture. The mixture was then kept at 65° C.under nitrogen and stirred for 22 hours at 300 rpm. The mixture was thenallowed to cool to room temperature and a dispersion of particles withsizes from 10 μm to 300 μm was obtained with a solids content ofapproximately 13% by weight.

Example 3

Polymeric micro gel particles were prepared in accordance with thefollowing procedure. A mixture of 75.5 g of a 40% solution of ammoniumsulfate in water, 7.5 g of acrylamide, 12.0 g of a 20% solution ofpolydiallyldimethylammonium chloride in water, 6.7 μL of poly(ethyleneglycol) diacrylate, and 20.0 g of de-ionized water was prepared in a 250mL three-neck flask equipped with a condenser and a mechanical stirrer.The mixture was purged with nitrogen gas for 30 minutes and then heatedto 35° C. A 5.0 g solution of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water (0.01%2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride by weight) wasthen injected into the mixture. The mixture was then kept at 35° C.under nitrogen and stirred for 22 hours at 300 rpm. The mixture was thenallowed to cool to room temperature and a dispersion of particles with asolid content of approximately 9% by weight was obtained. Thedispersions were then centrifuged at 4800 rpm for 5 minutes. Thesupernatant was then decanted and sediments were collected. Thesediments were then derived under vacuum at approximately 50° C.overnight to generate loose, well dispersed dried powders.

Example 4

Polymeric micro gel particles were prepared in accordance with thefollowing procedure. A mixture of 90.0 g of a mixture of t-butanol andwater (90% t-butanol by weight), 15.0 g of acrylamide, 0.50 g ofpoly(vinyl pyrrolidone), was prepared in a 250 mL three-neck flaskequipped with a condenser and a mechanical stirrer. The mixture waspurged with nitrogen gas for 30 minutes and then heated to 65° C. A 5.0g solution of azobisisobutyronitrile in ethanol (1%azobisisobutyronitrile by weight) was then injected into the mixture.After approximately 20 minutes a solution of 0.10 g ofN,N′-ethylene-bis-acrylamide in 5.0 g of a mixture of t-butanol andwater (90% t-butanol by weight) was injected into the mixture. Themixture was then kept at 65° C. under nitrogen and stirred for 22 hoursat 300 rpm. The mixture was then allowed to cool to room temperature anda dispersion of particles with sizes from 10 μm to 300 μm was obtainedwith a solid content of approximately 13% by weight. The dispersionswere then centrifuged at 4800 rpm for 5 minutes. The supernatant wasthen decanted and sediments were collected. The sediments were thendried under vacuum at approximately 50° C. overnight to generate loose,well dispersed dried powders.

Example 5

Polymeric micro gel particles were prepared in accordance with thefollowing procedure. A mixture of 75.5 g of a 40% solution of ammoniumsulfate in water, 7.5 g of acrylamide, 12.0 g of a 20% solution ofpolydiallyldimethylammonium chloride in water, 67 μl, of poly(ethyleneglycol) diacrylate, and 20.0 g of de-ionized water was prepared in a 250mL three-neck flask equipped with a condenser and a mechanical stirrer.The mixture was purged with nitrogen gas for 30 minutes and then heatedto 35° C. A 5.0 g solution of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water (0.01%2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride by weight) wasthen injected into the mixture. The mixture was then kept at 35° C.under nitrogen and stirred for 22 hours at 300 rpm. The mixture was thenallowed to cool to room temperature and a dispersion of particles wasobtained with a solid content of approximately 9% by weight. Thedispersions were then centrifuged at 4800 rpm for 5 minutes. Thesupernatant was then decanted and sediments were collected. Thesediments were then derived under vacuum at approximately 50° C.overnight to generate loose, well dispersed dried powders.

Example 6

Test composition 1 was prepared in accordance with the followingprocedure. A mixture of 75.5 g of a 40% solution of ammonium sulfate inwater, 7.5 g of acrylamide, 12.0 g of a 20% solution ofpolydiallyldimethylammonium chloride in water, and 20.0 g of de-ionizedwater was prepared in a 250 mL three-neck, flask equipped with acondenser and a mechanical stirrer. The mixture was purged with nitrogengas for 30 minutes and then heated to 35° C. A 5.0 g solution of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water (0.01%2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride by weight) wasthen injected into the mixture. The mixture was then kept at 35° C.under nitrogen and stirred for 22 hours at 300 rpm. The mixture was thenallowed to cool to room temperature and a dispersion of particles wasobtained with a solid content of approximately 9% by weight.

Example 7

Test composition 2 was prepared in accordance with the followingprocedure. A mixture of 75.5 g of a 40% solution of ammonium sulfate inwater, 7.5 g of acrylamide, 12.0 g of a 20% solution ofpolydiallyldimethylammonium chloride in water, 6.7 μL of poly(ethyleneglycol) diacrylate, and 20.0 g of de-ionized water was prepared in a 250mL three-neck flask equipped with a condenser and a mechanical stirrer.The mixture was purged with nitrogen gas for 30 minutes and then heatedto 35° C. A 5.0 g solution of2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water (0.01%2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride by weight) wasthen injected into the mixture. The mixture was then kept at 35° C.under nitrogen and stirred for 22 hours at 300 rpm. The mixture was thenallowed to cool to room temperature and a dispersion of particles with asolid content of approximately 9% by weight was obtained.

Example 8

Test composition 3 was prepared in accordance with the followingprocedure. A mixture of 90.0 g of a mixture of t-butanol and water (90%t-butanol by weight), 15.0 g of acrylamide, 0.50 g of poly(vinylpyrrolidone), was prepared in a 250 mL three-neck flask equipped with acondenser and a mechanical stirrer. The mixture was purged with nitrogengas for 30 minutes and then heated to 65° C. A 5.0 g solution ofazobisisobutyronitrile in ethanol (1% azobisisobutyronitrile by weight)was then injected into the mixture. After approximately 20 minutes asolution of 0.10 g of N,N′-ethylene-bis-acrylamide in 5.0 g of a mixtureof t-butanol and water (90% t-butanol by weight) was injected into themixture. The mixture was then kept at 65° C. under nitrogen and stirredfor 22 hours at 300 rpm. The mixture was then allowed to cool to roomtemperature and a dispersion of particles with sizes from 10 μm to 300μm was obtained with a solid content of approximately 13% by weight. Thedispersions were then centrifuged at 4800 rpm for 5 minutes. Thesupernatant was then decanted and sediments were collected. Thesediments were then derived under vacuum at approximately 50° C.overnight to generate loose, well dispersed dried powders.

Example 9

Test composition 4 was prepared in accordance with the followingprocedure. A mixture of 90.0 g of a mixture of t-butanol and water (90%t-butanol by weight), 15.0 g of acrylamide, 0.50 g of poly(vinylpyrrolidone), was prepared in a 250 mL three-neck flask equipped with acondenser and a mechanical stirrer. The mixture was purged with nitrogengas for 30 minutes and then heated to 65° C. A 5.0 g solution ofazobisisobutyronitrile in ethanol (1% azobisisobutyronitrile by weight)was then injected into the mixture. After approximately 20 minutes asolution of 0.40 g of N,N′-ethylene-bis-acrylamide in 5.0 g of a mixtureof t-butanol and water (90% t-butanol by weight) was injected into themixture. The mixture was then kept at 65° C. under nitrogen and stirredfor 22 hours at 300 rpm. The mixture was then allowed to cool to roomtemperature and a dispersion of particles was obtained with a solidcontent of approximately 13% by weight. The dispersions were thencentrifuged at 4800 rpm for 5 minutes. The supernatant was then decantedand sediments were collected. The sediments were then derived undervacuum at approximately 50° C. overnight to generate loose, welldispersed dried powders.

Example 10

Test composition 5 was prepared in accordance with the followingprocedure. A mixture of 90.0 g of a mixture of t-butanol and water (90%t-butanol by weight), 15.0 g of acrylamide, 0.50 g of poly(vinylpyrrolidone), was prepared in a 250 mL three-neck flask equipped with acondenser and a mechanical stirrer. The mixture was purged with nitrogengas for 30 minutes and then heated to 65° C. A 5.0 g solution ofazobisisobutyronitrile in ethanol (1% azobisisobutyronitrile by weight)was then injected into the mixture. After approximately 20 minutes asolution of 0.20 g of N,N′-ethylene-bis-acrylamide in 5.0 g of a mixtureof t-butanol and water (90% t-butanol by weight) was injected into themixture. The mixture was then kept at 65° C. under nitrogen and stirredfor 22 hours at 300 rpm. The mixture was then allowed to cool to roomtemperature and a dispersion of particles was obtained with a solidcontent of approximately 13% by weight. The dispersions were thencentrifuged at 4800 rpm for 5 minutes. The supernatant was then decantedand sediments were collected. The sediments were then derived undervacuum at approximately 50° C. overnight to generate loose, welldispersed dried powders.

Example 11

Test composition 6 was prepared in accordance with the followingprocedure. A mixture of 90.0 g of a mixture of t-butanol and water (90%t-butanol by weight), 15.0 g of acrylamide, 0.50 g of poly(vinylpyrrolidone), was prepared in a 250 mL three-neck flask equipped with acondenser and a mechanical stirrer. The mixture was purged with nitrogengas for 30 minutes and then heated to 65° C. A 5.0 g solution ofazobisisobutyronitrile in ethanol (1% azobisisobutyronitrile by weight)was then injected into the mixture. After approximately 20 minutes asolution of 0.050 g of N,N′-ethylene-bis-acrylamide in 5.0 g of amixture of t-butanol and water (90% t-butanol by weight) was injectedinto the mixture. The mixture was then kept at 65° C. under nitrogen andstirred for 22 hours at 300 rpm. The mixture was then allowed to cool toroom temperature and a dispersion of particles was obtained with a solidcontent of approximately 13% by weight. The dispersions were thencentrifuged at 4800 rpm for minutes. The supernatant was then decantedand sediments were collected. The sediments were then derived undervacuum at approximately 50° C. overnight to generate loose, welldispersed dried powders.

Example 12

Test composition 7 was prepared in accordance with the followingprocedure. Equal amounts of the powders from Examples 8, 9, 10, and 11were mixed to generate a powdered fluid loss control additive.

Example 13

Fluid loss tests were performed on test compositions 1-7 in accordancewith the following procedure. Varying dosages of each of the testcompositions as well as conventional fluid loss control additive N-DrillHT Plus® (commercially available from Halliburton Energy Services, Inc.,Houston, Tex.) were added to separate drilling mud compositionscomprising 0.09 g of viscosifier N-VIS® (commercially available formHalliburton Energy Services, Inc., Houston, Tex.), 14.0 g of bridgingagent BARACARB® 5, (commercially available from Halliburton EnergyServices, Inc., Houston, Tex.) 3.5 g of bridging agent CARACARB® 5(commercially available from Halliburton Energy Services, Inc., Houston,Tex.), and 137 g of a 26% by weight solution of NaCl in water. Theamount of NaCl in the drilling fluids that the fluid loss controladditives from Examples 6 and 7 were added to was reduced to account forthe brine present in those fluid loss control additives. The drillingfluids were each placed into a 175 mL “OFITE” HPHT Filter Press forfluid loss evaluation. In a typical experiment, pressures were adjustedto 600 and 100 psi (700 and 200 psi when the temperature is above 300°F.) on the top and bottom of a sample chamber, which was filled with mudand preheated to test temperature. Drained liquid, from a filter paperwith a porosity of 2.7 μm sealed on the bottom of the cylinder, wascollected. The performance of each fluid loss control additive isprovided below in Table 1. The acceptable performance by an additive wasset to be 10 mL volume of filtrate with 30 min test.

TABLE 1 Crosslinking Fluid Fluid Loss Control Temperature Agent DosageLoss Additive (° F.) (wt %) (wt %) (mL) N-DRILL HT PLUS ® 250 N/A 2.09.5 Test Composition 1 250 0 2.0 100 Test Composition 2 250 0.006 2.04.3 N-DRILL HT PLUS ® 300 N/A 2.0 100 Test Composition 2 300 0.006 2.0100 Test Composition 2 300 0.006 1.0 32 Test Composition 2 300 0.006 0.537 Test Composition 3 300 0.7 2.0 6.3 Test Composition 3 300 0.7 1.016.3 Test Composition 3 300 0.7 0.5 50.0 Test Composition 3 350 0.7 2.040.0 Test Composition 4 350 2.7 2.0 10.2 Test Composition 5 350 1.3 2.07.5 Test Composition 6 350 0.3 2.0 43.5 Test Composition 7 350 1.3 2.06.5

In some embodiments, the present invention provides a method comprising:providing an aqueous based treatment fluid comprising a fluid losscontrol additive that comprises at least one polymeric micro gel;placing the aqueous based treatment fluid in a subterranean formationvia a well bore penetrating the subterranean formation; allowing thefluid loss control additive to become incorporated into a filter cakelocated on a surface within the subterranean formation; allowing thefilter cake to be degraded; and producing hydrocarbons from theformation.

As can bee seen by Table 1, test composition 1 which represented a linerhigh molecular weight polymer did not show effectiveness as nearly allfluid drained out. Test composition 2 (in brine solution) was found toperform better than N-Drill HT Plus® (fluid loss of 4.3 mL compared to afluid loss of 9.5 mL) at the same dosage at 250° F. However, when testedat 300° F., both test composition 2 and N-DRILL HT Plus® nearly failedcompletely during the test at a dosage of 2 weight percent. Testcomposition 2 was able to reduce fluid loss to 32 mL and 37 mL at adosage of 1 weight percent and 0.5 weight percent, respectively.

At 300° F., test composition 3 (crosslinking agent content ofapproximately 0.7 weight percent) controlled fluid loss around 6.3 mL,however this additive failed to meet the criteria of acceptableperformance of 10 mL volume of filtrate within 30 minutes at 350° F.Test compositions 5 and 7 (crosslinking agent content of approximately1.3 weight percent) were able to control the fluid loss around 7.5 and6.5 mL at 350° F., respectively. However, increasing the crosslinkingagent content to 2.7 weight percent (as demonstrated by composition 4)resulted in fluid loss of 10.2 mL.

As can be seen by Table 1, and further demonstrated by FIG. 3, dosagehas an effect on the fluid loss control. As shown in FIG. 3, the fluidloss by test composition 3 decreased from 50 mL to 6.3 mL as the dosageof this additive increased from 0.5 wt percent to 2.0 weight percent.Thus, it is highly likely that the fluid loss would be further reducedat dosages higher than 2.0 weight percent, even though an optimal dosagemight exist.

As can further be seen by Table 1, the filtrate volume decreased from43.5 mL to 7.5 mL, when the crosslinking agent content of thecorresponding fluid loss additive increased from 0.3 weight percent to1.3 weight percent (See test compositions 6, 3, and 5). With a furtherincrease of the crosslinking agent content to 2.7 weight percent (seetest composition 4), the filtrate volume increased slightly to 10.2 mL.Surprisingly, synergist effect was observed when a mixture was used asthe additive (See test composition 7). The mud containing this additiveled to a fluid loss of only 6.5 mL, lower than those by any othernon-mixed test compositions and well below the acceptable value of 10mL. Without wishing to be limited to theory, it is believed that thesynergy was probably due to the physical distribution of the polymericmicro gel particles in the mud, which enhanced the fluid loss reduction.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the present invention. All numbers and ranges disclosed abovemay vary by some amount. Whenever a numerical range with a lower limitand an upper limit is disclosed, any number and any included rangefalling within the range are specifically disclosed. In particular,every range of values (of the form, “from about a to about b,” or,equivalently, “from approximately a to b,” or, equivalently, “fromapproximately a-b”) disclosed herein is to be understood to set forthevery number and range encompassed within the broader range of values.Moreover, the indefinite articles “a” or “an”, as used in the claims,are defined herein to mean one or more than one of the element that itintroduces. Also, the terms in the claims have their plain, ordinarymeaning unless otherwise explicitly and clearly defined by the patentee.

1-27. (canceled)
 28. A method comprising: providing a treatment fluidcomprising an aqueous base fluid and a fluid loss control additive,wherein the fluid loss control additive comprises a polymeric micro gel;placing the treatment fluid in a subterranean formation via a wellborepenetrating the subterranean formation; allowing the fluid loss controladditive to become incorporated into a filter cake located on a surfacewithin the subterranean formation; allowing the filter cake to bedegraded; and producing hydrocarbons from the subterranean formation.29. The method of claim 28, wherein the fluid loss control additive isincluded in the treatment fluid in an amount greater than 0% and up toabout 5% by weight of active component of the polymeric micro gel. 30.The method of claim 28, wherein the polymeric micro gel has a diameterrange of about 10 nm to about 1 mm.
 31. The method of claim 28, whereinthe polymeric micro gel comprises a reaction product formed by apolymerization reaction of a polymer or copolymer and a crosslinkingagent.
 32. The method of claim 31, wherein the polymer or copolymercomprises at least one unit based on at least one compound selected fromthe group consisting of: acrylamides, vinyl alcohols, vinylpyrrolidones,vinylpyridines, acrylates, polyacrylamides, polyvinyl alcohols,polyvinylpyrrolidones, polyvinylpyridines, polyacrylates, polybutylenesuccinate, polybutylene succinate-co-adipate,polyhydroxybutyrate-valerate, polyhydroxybutyrate-covalerate,polycaprolactones, polyester amides, polyethylene terephthalates,sulfonated polyethylene terephthalate, polyethylene oxides,polyethylenes, polypropylenes, aliphatic aromatic copolyester,polyacrylic acids, polysaccharides (such as dextran or cellulose),chitins, chitosans, proteins, aliphatic polyesters, polylactic acids,poly(glycolides), poly(ε-caprolactones), poly(hydroxy ester ethers),poly(hydroxybutyrates), poly(anhydrides), polycarbonates,poly(orthoesters), poly(amino acids), poly(ethylene oxides),poly(propylene oxides), poly(phosphazenes), polyester amides,polyamides, polystyrenes, a copolymer thereof, a homopolymer thereof, atetrapolymer thereof, and any derivative thereof.
 33. The method ofclaim 31, wherein the crosslinking agent comprises at least onecrosslinking agent selected from the group consisting of: diethyleneglycol dimethacrylate, azobisisobutyronitrile, and2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride.
 34. The methodof claim 28, wherein treatment fluid further comprises a bridging agent.35. The method of claim 28, wherein the aqueous base fluid is a highdensity brine with a density of at least about 9 lb/gal.
 36. A methodcomprising: providing a treatment fluid comprising an aqueous base fluidand a fluid loss control additive, wherein the fluid loss controladditive comprises a polymeric micro gel and a continuous medium,wherein the continuous medium comprises a solvent selected from thegroup consisting of an alcohol, a glycerine, a glycol, a polyglycolamine, a polyol, derivatives thereof, or combinations thereof; placingthe treatment fluid in a subterranean formation via a wellborepenetrating the subterranean formation; and allowing the fluid losscontrol additive to become incorporated into a filter cake located on asurface within the subterranean formation.
 37. The method of claim 36,further comprising: allowing the filter cake to be degraded.
 38. Themethod of claim 36, wherein the fluid loss control additive is includedin the treatment fluid in an amount greater than 0% and up to about 5%by weight of active component of the polymeric micro gel.
 39. The methodof claim 36, wherein the polymeric micro gel has a diameter range ofabout 10 nm to about 1 mm.
 40. The method of claim 36, wherein thecontinuous medium further comprises a co-solvent selected from the groupconsisting of fresh water, sea water, salt water, and brine.
 41. Themethod of claim 36, wherein the treatment fluid further comprises abridging agent.
 42. The method of claim 36, wherein the treatment fluiddoes not comprise a clay.
 43. A method comprising: providing a treatmentfluid comprising an aqueous base fluid and a fluid loss controladditive, wherein the fluid loss control additive comprises a polymericmicro gel, wherein the polymeric microgel is polymerized in a continuousmedium, and wherein the continuous medium comprises a solvent selectedfrom the group consisting of an alcohol, a glycerine, a glycol, apolyglycol amine, a polyol, derivatives thereof, or combinationsthereof; placing the treatment fluid in a subterranean formation via awellbore penetrating the subterranean formation; and allowing the fluidloss control additive to become incorporated into a filter cake locatedon a surface within the subterranean formation.
 44. The method of claim43, further comprising: allowing the filter cake to be degraded.
 45. Themethod of claim 43, wherein the continuous medium further comprises aco-solvent selected from the group consisting of fresh water, sea water,salt water, and brines.
 46. The method of claim 43, wherein thepolymeric micro gel has a diameter range of about 10 nm to about 1 mm.47. The method of claim 43, wherein the treatment fluid furthercomprises a bridging agent.
 48. The method of claim 43, wherein thetreatment fluid does not comprise clay.
 49. The method of claim 43,wherein the continuous medium further comprises a colloidal stabilizer.